440 THE BRAIN OF MAMMALS, BY TH. MEYNERT. 



that of the optic thalamus, and is formed by the unfolding of 

 the brachia of the corpora quadrigemina, and is the only fasci- 

 culus of the first link of the projection system that is freely 

 exposed on the surface of the cerebrum. The deep medullary 

 layer represents the origin of the tegmentum of the crus 

 cerebri. The two systems of medullary fibres decussate with 

 one another, and are therefore fused at the middle line. The 

 deep layer loses its compactness in its posterior fifth, in con- 

 sequence of its finer distribution through the grey matter 

 of the ganglion, and is no longer distinguishable as a special 

 medullary layer (fig. 250, Q). From the substance of the 

 bodies a processus corporis quadrigemini ad cerebellum is de- 

 veloped on either side (probably as a continuation of the 

 decussating fasciculi of the brachia), a name which however 

 by no means belongs to what usually bears it, the (so-named) 

 superjacent superior peduncle of the cerebellum, but only to the 

 frenulum, which is distinctly composed of two opposite halves 

 that pass through the valve of Vieussens to expand in the 

 superior vermiform process of the cerebellar medulla. 



The transverse section of the median medulla of the corpora 

 quadrigemina is identical with the medulla of the posterior 

 commissure (fig. 241, m'). The latter, however, proceeds not 

 only from the centre of origin, already mentioned, in the optic 

 thalamus, but is also continuous with the ganglion named the 

 pineal gland (conarium) and with the ganglion of its habenula. 

 The whole of this structure appears in longitudinal sections of 

 the cerebrum (fig. 241, m', Z, Z') as an S-shaped appendage of 

 the median medulla of the corpora quadrigemina, which is 

 composed (1) of a series of sections, in the form of an inferior 

 groove open posteriorly, constituting the posterior commis- 

 sure ; (2) of a superior groove placed upon this, and open 

 anteriorly, constituting the second curve of the S, the pineal 

 gland, with its medullary fasciculi arching parallel to the 

 surface ; and (3) of a beak of the S divided into two symme- 

 trical sections, the ganglion of the peduncle of the pineal 

 gland, forming with its longitudinal fasciculi, which, according 

 to Arnold, Jung, Luys, Inzani, and Lemoigne, are continuous 

 also with the fornix, the boundary between the optic thalamus 

 and the grey matter of the third ventricle. The symmetrical 



